Low temperature NMR spectroscopic investigation of a zinc bicarbonate compound: Thermodynamics of bicarbonate formation by insertion of CO2 into the zinc hydroxide bond of [TpBut,Me]ZnOH

Low temperature H-1 and C-13 NMR spectroscopic studies on solutions of the tris(3-t-butyl-5-methylpyrazolyl)hydroborato zinc hydroxide complex [Tp(But,Me)]ZnOH in the presence of CO2 have allowed for the identification of the bicarbonate moiety of [Tp(But,Me)]ZnOCO2H. For example, the bicarbonate ligand is characterized by H-1 and C-13 NMR spectroscopic signals at 13.2 and 161.9 ppm, respectively, in d(2)-dichloromethane solution at 189 K. Both [Tp(But,Me)]ZnOH and [Tp(But,Me)]ZnOCO2H may be observed in equilibrium in the presence of <1 atm of CO2 at 217 K, thereby allowing measurement of the equilibrium constant for insertion of CO2 into the Zn-OH bond. At 217 K, the equilibrium constant for formation of the bicarbonate complex is 6 +/- 2 x 10(3) M-1, corresponding to a value of Delta G = -3.8 +/- 0.2 kcal mol(-1). (c) 2011 Elsevier Ltd. All rights reserved.